1. Field of the Invention
This invention is related generally to an image processing method, and more particularly to a method for adjusting brightness of an image.
2. Description of Related Art
In photography, an exposure operation is typically performed through an auto exposure function to achieve the ideal brightness for an entire image. When under insufficient ambient lighting or when the exposure time is below a safe shutter speed, a flash lamp is employed for brightness compensation so that the captured image does not become unrecognizable due to inadequate brightness.
With currently available techniques, the intensity of each flash of the flash lamp is different. Moreover, the stability of the flash lamp is determined by the magnitude of the capacitance. As the capacitor of the flash lamp reduces in size, a discharge stability thereof deteriorates. With inadequate discharge stability, variations in the brightness of each captured image is generated. Due to the nonlinearity of the capacitor discharge, even with a large capacitance, variations in the stability result because of different discharge durations. Accordingly, the deteriorated stability in the pre-flash and main flash adversely affects the brightness level of the captured image. Therefore, when capturing a plurality of images, the inadequate stability of the flash lamp results in inconsistent brightness levels for each output image.
Flash lamp compensation typically utilizes an auto exposure parameter and a pre-flash image to estimate the main flash intensity. In a conventional compensation method, a sensitivity is first reduced, then the image is captured, and a digital gain is adjusted thereafter to perform compensation. Although this method may compensate the image brightness, discontinuities in brightness occurs when capturing a plurality of images. Moreover, since a sensitivity target value is reduced, the time for flash power up is shortened. As the time for the flash lamp to energize impacts the stability of the image, the shorter the time, the more unstable the image. For example, FIG. 1 is a schematic diagram of a brightness distribution after undergoing a conventional brightness compensation. In FIG. 1, the sensitivity is reduced to darken the image, and the digital gain is raised thereafter to perform compensation. As shown in FIG. 1, the compensation result improves as the expected image brightness reduces, although the discontinuity phenomenon becomes more apparent.